Case Studies

Offaly County Council

TEAM

Environmental Management Team

Sector

Environmental

Public Sector

Related Services

Environmental, Flood Risk Assessment

People To Contact

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Overview of service

The Commissioners of Public Works in Ireland have a number of regulatory functions under the 1945 Arterial Drainage Act, and S.I 122/2010 (Assessment and Management of Flood Risks) Regulations Section 50 requires that inter alia:

Offaly County Council instructed ORS to make a Section 50 application to the OPW for works at White Heart Lane in Portarlington.

Frequent flooding of the Blackstick stream affected access to White Heart Lane, which is extensively used as vehicular access to the nearby Colaiste Iosagain. The flooding results in significant traffic disruption and limited access to the school.

The proposed works consisted of the following alterations:

Upgrade stream dimensions to increase conveyance capacity
Upgrade 4 no. culverts along White Heart Lane

Flood Modelling

The existing regime was modelled using HEC-RAS software and site-specific survey data. Both the current flow estimation and the 100-year design flow were modelled.

As expected, the flood model showed significant overtopping of the current channels during a 1 in a 100-year flood event. The site-specific SECFRAM data support this.

The highest modelled water level during a 100-year event was estimated at 66.16 mOD. This is consistent and within the same magnitude as the SECFRAM study, which calculated a site-specific value of 66.28 mOD.

Project challenges

Flow Estimation

The nearest collection point of hydrometric data to the site in question is the Portarlington Station and Cushina Station, some 0.9 and 3.9 km away, respectively. Neither would provide sufficient information to calculate the existing flow as neither is located within the sub-catchment. The catchment is therefore classified as ungauged.

Ungauged flow estimation is considerably more difficult on smaller catchments as the variability in runoff characteristics (slope, soil, land use, surface drainage network) becomes more pronounced and has a more significant effect. The principal method used in flood peak estimation was deterministic.

Leaders

Key Contacts

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John Brennan

Managing Director
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Luke Martin

Associate and Joint Environmental Lead
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Alan Kiernan

Associate and Joint Environmental Lead

N5 Westport to Turlough Road

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